Many optical communication systems use modulation methods such as differential quadrature phase shift keying (DQPSK) for improving frequency usage efficiency. The DQPSK modulation method uses optical phase difference between two consecutive symbols. It uses four optical phase difference values such as 0, π/2, π, and 3π/2 to transmit two bits per one symbol. In a DQPSK optical reception circuit, a received signal passes π/4 delay interferometer and −π/4 delay interferometer to yield two bits of electric data signal per one symbol by means of photo diodes. Two bits of electric data signal are successfully recovered subject that the optical phase differences of delay interferometers are accurately adjusted to π/4 and −π/4, respectively.
However, such a conventional DQPSK optical reception circuit may fail to correctly recover data signals since the circuit may output multiple different types of demodulation signals.
A technique is proposed in which frame detection is carried out for every frame patterns that may be potentially output from the interferometers to select a correct demodulation signal from the outputted frame patterns.
FIG. 1 depicts a conventional DQPSK optical signal reception circuit 100. The optical reception module 100 depicted in FIG. 1 receives a DQPSK optical signal to demodulate it into data signals A and B with a DQPSK reception unit (reference numerals 101 through 133). The optical reception module 100 further de-multiplexes the data signals A and B into a data signal P with a demultiplexer 150. It is preferred that the data signals A and B outputted from the interferometers are a quadrature signal Q and an in-phase signal I, respectively. However, a set of the data signals A and B may not be a set of signal Q and signal I, respectively depending on the reception condition (demodulation condition) of the interferometers. The set of data signals A and B is uncertain and may be any one of multiple different combinations indicated in a table “DQPSK signal reception condition” in FIG. 1. In the table “DQPSK signal reception condition,” a double circle indicates a preferred reception condition, a single circle indicates a possible but not preferred reception condition, and a cross indicates an impossible reception condition.
A correct data signal (the preferred reception condition) may be acquired by processing the set of data signals A and B (the data signal P) outputted from the DQPSK optical reception module 100 with a frame processing circuit 170. The frame processing circuit 170 has a frame processing unit 171 to process frames, a frame synchronization circuit 172 to detect frames for all possible frame patterns to detect the reception condition, and a reception condition identification unit 173 to identify the reception condition.
The conventional arrangement depicted in FIG. 1 requires the combination of the optical reception module 100 and the frame processing circuit 170 for correct data signal demodulation. In addition, the frame synchronization circuit 172 becomes relatively large.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2007-20138    [Patent Document 2] Japanese Laid-open Patent Publication No. 2006-270909